Squashed liquid NMR sample tubes and RF coils

ABSTRACT

Nuclear magnetic resonance (NMR) systems and methods employing squashed (transversely-elongated) sample vessels (sample tubes or flow cells) for holding liquid NMR samples, and matching squashed saddle-shaped RF coils allow reducing sample and/or coil losses and increasing the RF circuit quality factors (Q). In a present implementation, the RF coils and sample vessels have rectangular cross-sections. Rounded (e.g. ellipsoidal) or other squashed cross-sections may also be used. The coil corresponding to the highest sample losses is positioned such that the magnetic field generated by the coil is along the major axis of the sample vessel. Squashed sample vessels may also be used with conventional circular coils, particularly for low-temperature measurements where sample losses dominate.

FIELD OF THE INVENTION

The invention in general relates to nuclear magnetic resonance (NMR),and in particular to radio-frequency (RF) coils, coil assemblies, andsample vessels for NMR.

BACKGROUND OF THE INVENTION

Nuclear magnetic resonance (NMR) spectrometers typically include asuperconducting magnet for generating a static magnetic field B₀, andone or more special-purpose radio-frequency (RF) coils for generating atime-varying magnetic field B₁ perpendicular to the field B₀, and fordetecting the response of a sample to the applied magnetic fields. EachRF coil can resonate at the Larmor frequency of a nucleus of interestpresent in the sample. The resonant frequency of interest is determinedby the nucleus of interest and the strength of the applied staticmagnetic field B₀. The RF coils are typically provided as part of an NMRprobe, and are used to analyze samples situated in test tubes or flowcells. Conventional NMR magnets and RF coils are characterized bycylindrical symmetry. The direction of the static magnetic field B₀ iscommonly denoted as the z-axis, while the plane perpendicular to thez-axis is commonly termed the x-y or θ plane. In the followingdiscussion, the term “longitudinal” will be normally used to refer tothe z-direction, while the term “transverse” will be used to refer tothe x and y directions.

Conventional RF coils used for NMR include helical coils, saddle coils,and birdcage resonators. Conventional RF resonators employed in liquidNMR spectroscopy are designed with care to produce a largecylindrically-symmetric volume of RF magnetic field homogeneity. Forinformation on various NMR systems and methods see for example U.S. Pat.Nos. 4,398,149, 4,388,601, 4,517,516, 4,641,098, 4,692,705, 4,840,700,5,192,911, 5,818,232, 6,201,392, 6,236,206, and 6,285,189.

An NMR probe can include multiple NMR coils, each tuned for performingNMR measurements on a different nucleus of interest. For example, an NMRprobe can include one coil for performing NMR measurements on protons,and another coil for performing NMR measurements on other nuclei ofinterest, such as ¹³C or ¹⁵N. In such an NMR probe, the design of onecoil can affect the performance of the other coil(s). In order to reducethe coupling between two coils, the coils can be disposed in aquadrature configuration, so that the magnetic fields generated by thecoils are mutually orthogonal. This configuration minimizes the mutualinductance between the coils.

The measurement sensitivity that can be achieved with an NMR coilincreases with the coil quality factor Q and its filling factor n. Thequality factor Q can be maximized by reducing coil and sample losses.The filling factor n can be increased by reducing the coil size relativeto the sample. At the same time, reducing the coil size relative to thesample can increase magnetic field inhomegeneities. Inhomogeneities inthe RF magnetic field adversely affect the measurement sensitivity.Moreover, the coil design and dimensions are constrained by therequirement that the coil resonate in a desired frequency range.

In U.S. Pat. No. 5,552,709, Anderson proposed a method of reducingelectrical losses in the sample through the use of special sample cellsthat compartmentalize the sample. The volume of a cylindrical samplecell is broken up into a plurality of tubular compartments separated byelectrically-insulating material. The insulating material reduces theelectrical current paths within the sample. The transversecross-sections of the partitions can be shaped as circles, triangles,rectangles, squares, or sections of cylindrical shells. The describedmethod can be difficult to implement in practice.

SUMMARY OF THE INVENTION

The present invention provides transversely-elongated (squashed) samplevessels, radio-frequency coils, coil supports, coil assemblies, andassociated systems and methods allowing reduced RF losses and improvedNMR measurement sensitivities. Each sample vessel preferably has anelongated transverse shape matching the transverse shape of thecorresponding coil(s). Multiple nested orthogonal coils may be used. Thelong axis of the sample vessel transverse cross-section preferablycoincides with the direction of the RF magnetic field corresponding to areceiver coil of interest. In the preferred embodiment, a saddle-shapedradio-frequency coil for an NMR probe and spectrometer includes aplurality of longitudinal conductors; and a plurality of generallytransverse conductors connected to the longitudinal conductors so as togenerate a radio-frequency magnetic field oriented along a firsttransverse direction, wherein a transverse cross-section of the coilformed by the longitudinal conductors and transverse conductors has agenerally-squashed (elongated) shape having a larger extent along thefirst transverse direction than along a second transverse directionperpendicular to the first transverse direction. The first transversedirection coincides with the direction of the RF magnetic fieldgenerated by the coil. The coil may have rectangular, rounded (e.g.ellipsoidal) or other squashed (elongated) cross-sections. The directionof the RF magnetic field coincides with the long transverse axis of thesample vessel. Squashed sample vessels may be used with conventionalcoils having cylindrical symmetry, particularly for low-temperatureapplications.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and advantages of the present invention willbecome better understood upon reading the following detailed descriptionand upon reference to the drawings where:

FIG. 1-A is a schematic diagram of an exemplary NMR spectrometeraccording to the present invention.

FIG. 1-B is a schematic diagram of an exemplary NMR probe of thespectrometer of FIG. 1-A according to the present invention.

FIGS. 2-A and 2-B show isometric views of two exemplarystationary-sample vessel geometries of the present invention.

FIGS. 3-A–C show sectional views of exemplary flow cell configurationsaccording to the present invention.

FIG. 4-A shows an isometric view of a conductive central part of anexemplary squashed RF coil having a rectangular transversecross-section, according to a preferred embodiment of the presentinvention.

FIG. 4-B shows an isometric view of a dielectric support for supportingthe conductive part of FIG. 2-A.

FIG. 4-C shows an isometric view of two capacitance-enhancing ringssuitable for use with the conductive part and dielectric support ofFIGS. 2-A–B.

FIG. 5 shows an isometric view of a conductive central part of anexemplary squashed RF coil having a rectangular transversecross-section, according to another preferred embodiment of the presentinvention.

FIG. 6-A shows an isometric view of a saddle-shaped conductive part ofan exemplary coil having a rectangular transverse cross-section,suitable for use in conjunction with a coil such as the coilsillustrated in FIGS. 4-A and 5, according to the present invention.

FIG. 6-B shows an isometric view of a support for supporting theconductive part of FIG. 6-A, according to the present invention.

FIGS. 7-A–B show transverse sectional views of two exemplary NMR probegeometries according to the present invention.

FIG. 8 illustrates in a transverse sectional view computed magneticfield lines generated in an exemplary coil configuration characterizedby cylindrical symmetry, according to the present invention.

FIGS. 9-A–B show computed magnetic field lines generated in an exemplarycoil configuration comprising two coaxial, mutually-orthogonal RF coilscharacterized by rectangular symmetry, according to the presentinvention.

FIG. 9-C shows computed magnetic field lines generated by an exemplarycoil characterized by ellipsoidal symmetry, according to the presentinvention.

FIG. 10 shows a nutation plot for an exemplary NMR probe including arectangular RF coil and sample vessel, according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, it is understood that each recited elementor structure can be formed by or be part of a monolithic structure, orbe formed from multiple distinct structures. A stationary-sample vesselis a vessel such as a test tube through which the sample does notordinarily flow. A sample may be moved, for example spun, in astationary-sample vessel. The statement that a coil is mechanicallycoupled to a sample holder is understood to mean that the coil is heldin fixed relation to the sample holder. Such a coil may be directlymounted on the sample holder, indirectly coupled to the sample holderthrough intermediate support structures. The statement that a first coilis positioned orthogonally with respect to a second coil is understoodto mean that the magnetic fields generated by the first and second coilsare substantially orthogonal. The statement that a first coil ispositioned concentrically with respect to the second coil is understoodto mean that the two coils have substantially coincident longitudinalaxes. The term “ring” encompasses slotted (discontinuous) rings. Unlessexplicitly specified otherwise, the term “low temperature” is understoodto refer to temperatures lower than 200 K. Unless explicitly limitedotherwise, the statement that an NMR coil is “saddle-shaped” isunderstood to mean that the coil is shaped to generate a magnetic fieldalong a single transverse direction perpendicular to the long axis ofthe NMR sample and to the long axis of the homogeneous region of thestatic magnetic field. Saddle-shaped coils are distinct from birdcagecoils, which are typically driven in quadrature to generate magneticfields alternatively along orthogonal directions, and from solenoids.The statement that a coil is used to perform a nuclear magneticmeasurement on a sample is understood to mean that the coil is used astransmitter, receiver, or both.

The following description illustrates embodiments of the invention byway of example and not necessarily by way of limitation.

Conventional NMR sample vessels for holding liquid NMR samples andassociated RF coils are characterized by cylindrical symmetry. Thepresent invention relies on the observation that the signal-to-noiseratio (SNR) achievable in NMR systems can be improved through the use ofappropriately-oriented transversely-squashed sample tubes and,optionally, matching transversely-squashed saddle-shaped RF coils. TheSNR improvements can be particularly noticeable where the dominant sinkfor power dissipation in the NMR probe is the NMR sample itself, as forexample in the spectroscopy of high ionic strength aqueous samples inhigh-Q probes.

Consider a radio-frequency magnetic field B oriented along the y-axis:B _(rf) =B ₁ cos(ωt)ŷ.  [1]If the coil is adequately designed, the radio-frequency electric fieldinside the sample has the form:E _(rf) =−xB ₁ω sin(ωt){circumflex over (z)}.  [2]Consider a sample vessel of x and y dimensions a and b, respectively,with b the larger dimension aligned with the RF magnetic field (y-axis).The NMR signal is proportional to the cross-sectional area of the samplevessel, ab. The noise in the NMR experiment is proportional to thesquare root of the power dissipated during a transmitted RF pulse.Consider now a liquid NMR sample for which conduction losses in thesample dominate the dissipated RF power, as is the case typically inlow-temperature probes or for salty aqueous samples in high-Q probes.The dissipated power isP∝∫σE ² dxdy∝σba ³,  [3]where σ is the sample conductivity, and the integral is carried out overthe cross-section of the interior of the sample vessel. Thesignal-to-noise ratio is then

$\begin{matrix}{{S/N} \propto \frac{ab}{\sqrt{{ba}^{3}}} \propto {\sqrt{\frac{b}{a}}.}} & \lbrack 4\rbrack\end{matrix}$

Eq. [3] shows that generally the signal-to-noise ratio (SNR) willimprove with larger b and lower a. In practice, increasing the aspectratio b/a too much may lead to unacceptable loss of sample volume orreductions in the homogeneity of the static and RF magnetic fields.Increasing the aspect ratio b/a may also not be sufficiently beneficialonce the power dissipated in the sample becomes comparable to the powerdissipated in the RF coils. Generally, an optimal aspect ratio for agiven application can be determined/tested empirically.

Employing RF coils having squashed shapes matching the sample vesselshapes allows improving the filling factor of the coils. The coils andsample vessels are preferably arranged such that the magnetic fieldcorresponding to the receive coil is aligned with the major axis of thesample vessel. In a typical arrangement, where the proton channel isused for detection, the receive (proton) coil also corresponds to thehighest-sample-loss frequency. The sample losses at the proton resonantfrequency are often higher than those at the resonant frequencies ofother nuclei (e.g. carbon), since the proton measurements are performedat higher RF frequencies. In such an application, the sample vessel ispreferably arranged such that its major axis is aligned with the protonRF magnetic field. If a different nucleus (e.g. a carbon) is used forthe receive channel, the RF coils and sample vessels may be aligned tominimize losses at the receive channel resonant frequency, even ifsample losses are higher at a different frequency.

Squashed birdcage and gradient coils have been used in the past in MRIapplications, in order to fit the coils around non-symmetrical objectssuch as human body, as described for example in U.S. Pat. Nos. 5,510,714and 6,479,998. Squashed sample cells and resonant cavities have alsobeen proposed for use in ESR (Electron Spin Resonance), as described forexample by Poole in “Electron Spin Resonance: A Comprehensive Treatiseon Experimental Techniques,” Wiley Interscience, John Wiley & Sons, NewYork, 1983. Electron spin resonance involves measuring the spinresonance of electrons, which typically occurs at much higherfrequencies than nuclear magnetic resonance.

FIG. 1-A is a schematic diagram illustrating a nuclear magneticresonance (NMR) spectrometer 12 according to a preferred embodiment ofthe present invention. Spectrometer 12 comprises a magnet 16, an NMRprobe 20 inserted in a cylindrical bore of magnet 16, and acontrol/acquisition system 18 electrically connected to magnet 16 andprobe 20. One or more shim coils 32 are coupled to magnet 16. Shim coils32 may be provided as part of magnet 16, particularly if shim coils 32are superconducting cryo coils maintained at a low temperature. Shimcoils 32 may also be provided separately from magnet 16, particularly ifshim coils 32 are adjustable room-temperature coils. Shim coils 32 canbe characterized by cylindrical symmetry. One or more gradient coils canbe provided as part of probe 20, for generating a magnetic fieldgradient.

Probe 20 includes a conventional sample holder or sample holdingcomponents for holding one liquid NMR sample of interest 22 at a time,while measurements are performed on sample 22. Probe 20 can be aflow-through probe or a stationary-sample probe. In a flow-throughprobe, sample 22 is typically held in a flow cell. In astationary-sample probe, sample 22 is typically held in a test tube. Ina stationary-sample probe the sample may be spun around the test tubelongitudinal axis, as is known in the art. Probe 20 further includes acoil assembly having one or more saddle-shaped radio-frequency (RF)coils 30 a–b. Generally, probe 20 may include more than two RF coils.Probe 20 can be a low-temperature probe or a room temperature probe. Ina low-temperature probe, coils 30 a–b are normally held at a lowtemperature, e.g. at a temperature lower than the boiling point ofnitrogen (78 K).

Magnet 16 applies a static longitudinal magnetic field B₀ to sample 22.Shim coils 32 apply a compensatory static magnetic field that reducesthe spatial inhomogeneity of the static magnetic field B₀ applied tosample 22. Control/acquisition system 18 comprises electronic componentsfor applying desired radio-frequency pulses to probe 20, and acquiringdata indicative of the nuclear magnetic resonance properties of thesamples within probe 20. Coils 30 a–b are used to apply transverseradio-frequency magnetic fields B₁ to sample 22, and/or to measure theresponse of sample 22 to the applied magnetic fields. The same coil canbe used for both applying an RF magnetic field and for measuring thesample response to the applied magnetic field. Alternatively, one coilmay be used for applying an RF magnetic field, and another coil formeasuring the response of the sample to the applied magnetic field. In acommon application, sample 22 is excited at multiple frequencies usingmultiple coils 30 a–b, while a signal (e.g. a proton signal) is read atsingle frequency using only one of coils 30 a–b (e.g. inner coil 30 a).

Each saddle-shaped RF coil 30 a–b is electromagnetically coupled tosample 22, and is electrically connected to control/acquisition system18. RF coils 30 a–b are preferably disposed concentrically relative tothe longitudinal axis of probe 20. RF coils 30 a–b are disposed mutuallyorthogonally, in order to generate mutually orthogonal transversemagnetic fields. Each RF coil 30 a–b can be mounted on a support made ofa material such as quartz or sapphire, chosen not to affect the NMRmeasurements of interest. Each coil 30 a–b is preferablysusceptibility-compensated, so as to match the magnetic susceptibilityof its environment (typically air of vacuum).

Each RF coil 30 a–b can have a different resonant frequency, which canbe chosen to match the Larmor frequency of a nucleus on interest. Forexample, one of coils 30 a–b can be tuned to perform ¹H NMRmeasurements, while the other can be tuned to perform ¹³C NMRmeasurements. Coils 30 a–b can be used in general to perform NMRmeasurement on other nuclei of interest, such as ¹⁹F and ³¹P. In apresent implementation, an inner patterned-foil coil 30 a as describedbelow is used to perform ¹H NMR measurements, while an outer orthogonalwire coil 30 b is used to perform ¹³C NMR measurements. In alternativeimplementations, both coils may be formed from patterned foils or wires.The resonant frequencies of RF coils commonly used withcommercially-available NMR magnets range from about 10 MHz to about 1GHz. Proton resonant frequencies are typically on the order of hundredsof MHz. Resonant frequencies for other nuclei can be lower.

FIG. 1-B shows a more detailed view of probe 20. As shown, probe 20 mayinclude low-temperature control components such as a Dewar 33 forholding coils 30 a–b at a low temperature. The low-temperature controlcomponents preferably allow maintaining sample 22 at a desired sampletemperature, for example room temperature. Probe 20 can also includesample temperature components for controlling the temperature of sample22. The sample temperature can be controlled for example by flowing agas (e.g. air) longitudinally along the sample walls. The outer surfaceof probe 20 preferably has a cylindrical shape matching thecorresponding cylindrical shape of the magnet bore. Internal surfaceswithin probe 20, such as the inner surface of Dewar 33, may haveelongated transverse cross-sections matching the cross-section of thesample vessels of interest.

FIGS. 2-A and 2-B show isometric views of two NMR stationary-sampletubes 122, 222 respectively, according to the present invention. Sampletubes 122, 222 are shaped overall as transversely-squashed tubes havinguniform transverse cross-sections. Sample tube 122 has agenerally-rectangular transverse cross-section, while sample tube 222has a generally-ellipsoidal transverse cross-section. Sample tube 122has a five-sided (four planar lateral sides and one bottom) wall 124which encloses a sample-holding volume 130. Wall 124 defines arectangular sample tube opening 128 used for inserting and removing thesample of interest to and from sample tube 122. Sample tube 224 has acorresponding ellipsoidal sample tube opening 228. Sample-holding volume130 is bounded longitudinally by two squashed plugs 126 a–b situated onopposite sides of sample-holding volume 130. Plugs 126 a–b can be usedto center the sample of interest within coil 30 a, when the volume ofthe sample of interest is lower than the total volume contained withinwall 124. Plugs 126 a–b are preferably magnetic susceptibility-matchedto the liquid sample of interest. Sample tube 222 may include similarplugs having ellipsoidal cross-sections. Sample tubes 122, 222 arepreferably used with corresponding RF coils and other components havingmatching shapes and aspect ratios, as described below.

FIG. 3-A shows a schematic longitudinal sectional view of a flow cell322 according to the present invention. Flow cell 322 defines a squashedsample-holding volume 330 for holding the sample of interest. A sampleinlet 328 a and a sample outlet 328 b are situated on opposite sides ofsample-holding volume 330. FIGS. 3-B and 3-C illustrate two suitableshapes for the transverse cross-section of flow cell 322 alongsample-holding volume 330. FIG. 3-B shows a flow cell 422 having agenerally rectangular transverse cross-section, while FIG. 3-C shows aflow cell 522 having a generally ellipsoidal transverse cross section.

The transverse aspect ratios (the ratio of the two transversedimensions) of sample vessels such as the vessels 122, 222, 322 arepreferably between 1.2 and 10, more preferably between 1.5 and 8, andideally between 2 and 4. In a present implementation, the transverseinner dimensions of the NMR sample vessels are about 2 mm and 7 mm. Fora given sample vessel maximum transverse size, increasing the transverseaspect ratio can allow reduced RF sample losses, as discussed in detailbelow. At the same time, increasing the aspect ratio too much can leadto unacceptable loss of sample volume and thus signal strength.

The longitudinal extent (length) of the sample vessels is preferablybetween 4 and 12 inches (10–30 cm), more preferably between 6 and 10inches (15–25 cm). In a present implementation, the sample vessels areabout 8 inches (20 cm) long. Employing too short a sample vessel canlead to suboptimal alignment of the sample vessel within the NMR probe,increased inhomogeneities in the flow of heating gas over the samplevessel, and shimming difficulties. Inhomogeneities in heating gas flowcan lead to spatial inhomogeneities in the sample temperature, whilesuboptimal shimming can lead to spatial inhomogeneities in the staticmagnetic field. The sample-holding volume defined by the sample vesselsis preferably on the order of several cm long. The sample vessels arepreferably made of NMR-compatible materials such as quartz, sapphire, orborosilicate glass (e.g. pyrex®).

Squashed sample vessels as described above are preferably used with RFcoils and associated components having matching cross-sections, asillustrated below. Using matching cross-sections for the coils andsample vessels allows achieving improved filling factors, whichgenerally allows improved signal-to-noise ratios (SNR). Squashed samplevessels may also be used with conventional, cylindrical RF coils andassociated components, particularly for low-temperature applicationswhere sample losses dominate. In such applications, coil losses arerelatively low, and improved SNRs for NMR measurements can be achievedwith squashed sample vessels even if the coil filling factors are notmaximized.

FIG. 4-A shows an isometric view of an electrically-conductive centralstructure 32 of squashed (transversely-elongated) saddle-shapedradio-frequency (RF) coil 30 a according to a presently preferredembodiment of the present invention. Central structure 32 has an overallshape of a transversely-squashed tube matching the shape of the samplevessel employed with central structure 32. Saddle-shaped coil 30 agenerates an RF magnetic field directed along a single direction 54perpendicular to the static magnetic field B₀ and to the longitudinalaxis of the sample. Direction 54 is perpendicular to the direction ofthe RF magnetic field generated by saddle-shaped RF coil 30 b (shown inFIGS. 1-A–B). Central structure 32 is mounted on a squashed dielectricsupport 60, illustrated in FIG. 4-B. Coil 30 a includes centralstructure 32, dielectric support 60, and a pair of squashed, conductivecapacitance-enhancing floating shields 34 a–b shown in FIG. 4-C. Shields34 a–b are disposed on opposite longitudinal sides relative to thecenter of central structure 32. The central axis of each shield 34 a–bis aligned with the longitudinal central axis of central structure 32.Central structure 32 and shields 34 a–b are preferably disposed onopposite sides of support 60. For example, central structure 32 can bemounted on the outside of support 60, and shields 34 a–b can be mountedon the inside of support 60. Shields 34 a–b may be continuous, as shownin FIG. 4-C, or may include longitudinal slots for increasing thepenetration of the orthogonal RF magnetic field generated by RF coil 30b. For information on cylindrical shields described for use with priorart cylindrical coils see for example Hill et al., U.S. Pat. No.5,192,911, and Behbin, U.S. Pat. No. 6,008,650.

A squashed measurement volume 36 is defined in the center of centralstructure 32, between shields 34 a–b. The measurement volume 36sequentially accommodates NMR samples of interest held in squashedsample tubes or flow cells. Shields 34 a–b are capacitively coupled tocentral structure 32 along at least part of the surfaces of shields 34a–b adjacent to measurement volume 36. Shields 34 a–b can be used toreduce the parasitic excitation of the NMR samples due to RF pickup fromcoil leads or other conductive structures, and to shield the NMR samplesfrom undesired external electric fields. Shields 34 a–b also provideadditional distributed capacitance to coil 30 a.

Central structure 32 includes a pair of external longitudinal leads 38a–b extending longitudinally away from measurement volume 36. Leads 38a–b provide an electrical connection to control/measurement electronics18 (shown in FIG. 1). Central structure 32 further includes a pair ofsquashed transverse conductors (rings) 40 a–b, and a set of longitudinalconductors 52 interconnecting rings 40 a–b. In a transversecross-section, the positions of conductors 52 preferably define anelongated perimeter. Rings 40 a–b are disposed on opposite sides ofmeasurement volume 36. A plurality of elongated, longitudinal conductivesegments (rungs, conductors) 52 interconnect rings 40 a–b. Longitudinalsegments 52 extend along measurement volume 36 between rings 40 a–b. Inthe configuration shown in FIG. 4-A, ring 40 b includes a pair oflongitudinal slots 41 facing direction 54.

Rings 40 a–b and segments 52 form two loops facing each other along atransverse (x- or y-) direction, for generating an RF magnetic fieldalong direction 54. Current flows through the two loops in the samedirection (clockwise or counterclockwise), such that the RF magneticfields generated by the two loops reinforce each other. The RF currentin each loop flows around a corresponding coil window 53 facingdirection 54. As an external voltage is applied over leads 38 a–b,current flows from lead 38 a through ring 40 a–b and longitudinalsegments 52 to lead 38 b. The alternating current flowing around ring 40b is capacitively coupled across longitudinal slots 41 by shield 34 b.The current flow through coil 30 a generates a transverse RF magneticfield along direction 54.

A plurality of pairs of elongated, generally longitudinal slots 58 a–care formed between segments 52. A central pair of slots 58 a extendsbetween segments 52 and through ring 40 a. Slots 58 a–c face atransverse direction orthogonal to the direction 54. The two slots ineach pair are disposed symmetrically with respect to the central axisparallel to direction 54. Slots 58 a–c serve to reduce the obstacleformed by conductors 52 to the magnetic field lines generated by theorthogonal coil 30 b. Preferably, the approximate location and number ofslots 58 a–c are chosen so as to provide improved magnetic fieldhomogeneity and increased penetration of the magnetic field generated byorthonogal coil 30 b.

The shape and orientation of slots 58 a–c are preferably chosen suchthat the local orientation of slots 58 a–c tracks the local direction ofRF current flow within coil 30 a, so as to minimize the impact of slots58 a–c on the RF current passing through coil 30 a, and thus maintain ahigh Q factor for coil 30 a. Preferably, a pair of central slots 58 aextend longitudinally from the inner side of ring 40 b, along volume 36and ring 40 a, to the outer edge of ring 40 a. In general, slots such asslots 58 a–c may extend over ring 40 b, to the outer side of ring 40 b.Two pairs of lateral slots 58 b–c extend longitudinally along centralvolume 36, and then curve transversely (horizontally in FIG. 4-A) alongthe inner side of ring 40 a. The curvature of slots 58 b–c is chosen totrack the local direction of current flow through segments 52 and aroundring 40 a. It was observed that in an arrangement such as the one inFIG. 4-A, in which ring 40 a does not include longitudinal slots facingdirection 54, making lateral slots 58 b–c curve around the RF magneticfield window allows achieving reduced RF coil losses and improved Qvalues.

Central structure 32, which includes rings 40 a–b and longitudinalconductors 52, is preferably formed from a singlesusceptibility-compensated thin sheet. For example, central structure 32and shields 34 a–b may be made of susceptibility-compensatedpalladium-plated copper about 0.002 inches thick. In general, othermaterials such as aluminum, platinum, copper and stacks of suchmaterials are suitable for central structure 32 and shields 34 a–b. Forexample, an Al—Cu susceptibility-compensated sandwich may be used. Othermaterials having susceptibilities of opposite signs may be used to yielda magnetic susceptibility for coil 30 a equal to the magneticsusceptibility of air or vacuum. To make coil 30 a, a suitable patternis first cut by well-known methods into a flat sheet of a desiredmaterial or materials. The flat pattern is then disposed around a glassor quartz tube to form the squashed rings 40 a–b of coil 30 a.

In a present implementation, the overall transverse size of rings 40 a–band shields 34 a–b is on the order of 1 cm, and the transverse size ofeach longitudinal conductor 52 is about 1 mm. For typical NMRapplications, transverse coil sizes for coil 30 a can range from a fewmillimeters to a few centimeters, and the transverse sizes oflongitudinal conductors 52 can range from tenths of mm to a few mm. Thelongitudinal extents of longitudinal conductors 52 and rings 40 a–b canbe on order of a few cm.

FIG. 4-B shows an isometric view of a dielectric coil support 60suitable for supporting the conductive part of coil 30 a. Support 60 isshaped as a hollow shell, and has a longitudinal inner bore 66 forreceiving the sample tubes or flow cells of interest. Inner bore 66extends from the top end to the bottom end of support 60. Support 60 hasa longitudinal outer surface 62, and a longitudinal inner surface 64.Shields 34 a–b (shown in FIG. 2-C) are attached to and abut innersurface 64. Central structure 32 (shown in FIG. 2-A) is attached to andabuts outer surface 62. Support 60 is preferably made of a dielectricmaterial that does not interfere with NMR measurements, such as quartzor sapphire. Support 60 is secured at its longitudinal ends to thestructure of the probe containing coil 30 a. Support 60 may be formedfrom a single monolithic piece, or from four assembled planar pieces.

The transverse cross-sections of central structure 32, support 60, andshields 34 a–b are preferably generally shaped as rectangles matchingthe aspect ratio of the sample vessels to be used. In alternativeembodiments, the transverse cross-sections may have generally-roundedshapes, for example ellipsoidal shapes. The aspect ratios (ratio oflength to width) of the general shapes are preferably between about 2:1and 4:1. Lower or higher aspect ratios, such as 1.2:1–1:5:1, 1.5:1–2:1,or 4:1–10:1, may be used.

FIG. 5 shows an isometric view of a central conductive part 132 of an RFcoil according to another preferred embodiment of the present invention.The RF magnetic field generated by the coil is oriented along direction54. Central part 132 can be used in conjunction with a dielectricsupport and capacitance-enhancing shield such as the ones shown in FIGS.4-B–C. Central part 132 includes two conductive rings 140 a–b disposedon opposite longitudinal sides of a measurement volume 136. Rings 140a–b are interconnected by a plurality of longitudinal conductivesegments 152. Each ring 140 a–b includes a pair of longitudinal slots141 a–b, respectively. Slots 141 a–b face direction 54.

A plurality of pairs of longitudinal slots 158 a–c are defined betweensegments 152 and through ring 140 a. Longitudinal slots 158 a–c extendlinearly from the inner side of ring 140 b, along volume 136 and ring140 a, to the outer edge of ring 140 a. Linear slots such as the onesshown in FIG. 5 are preferably used in RF coils which include slots 141a. It was observed that such linear slots match the actual RF currentdistribution within conductive part 132, thus allowing reduced coillosses.

FIG. 6-A shows a schematic isometric view of the conductive part of amulti-turn saddle-shaped RF coil 30 b suitable for use in a coilassembly in conjunction with coil 30 a (shown in FIGS. 4-A–C). Coil 30 bis preferably disposed coaxially with coil 30 a. The conductor of FIG.6-A can be formed from a conventional susceptibility-compensated wire,which loops around an RF magnetic field window 53′. The direction of thetransverse RF magnetic field generated by coil 30 b is illustrated bythe arrow 78. The magnetic field generated by coil 30 b is orthogonal tothe magnetic field generated by coil 30 a (illustrated at 54 in FIG.4-A). Similarly, magnetic field window 53′ is orthogonal to the window53 of coil 30 a.

FIG. 6-B shows an isometric view of a coil support 82 suitable forsupporting the conductor of coil 30 b. Coil support 82 includes twoparallel transverse mounting plates 86 a–b, and a plurality oflongitudinal rods 88 connecting mounting plates 86 a–b. Mounting plates86 a–b have corresponding aligned central apertures defining an internalcoil space 90 for receiving coil 30 a and the sample of interest.Longitudinal rods 88 are arranged around the central apertures ofmounting plates 86 a–b. The positions of longitudinal rods 88 define asquashed perimeter. Each longitudinal conductor of the wire shown inFIG. 6-A is attached to one of the longitudinal rods 88 at one or morepoints. Mounting plates 86 a–b are secured to the structure of the probecontaining coil 30 b.

Preferably, in order to optimize the filling factor of coil 30 b and theuse of space within the NMR probe, coil 30 b generally matches coil 30 ain overall transverse shape and aspect ratio. For example, if coil 30 ahas a rectangular cross-section, coil 30 b is preferably chosen to havea rectangular cross-section of a similar aspect ratio. If coil 30 a hasan elliptical cross-section, coil 30 b is preferably chosen to have anelliptical cross-section. In general, however, coil 30 b may have acircular cross-section or some other cross-section that does not matchthat of coil 30 a.

Aligning the direction of the RF magnetic field generated by coil 30 bwith the minor axis of the squashed sample vessel can lead to increasedsample losses at the resonant RF frequency of coil 30 b. Such samplelosses may have diminished significance if coil 30 b is only used forsample excitation and not detection, and if the sample losses at theresonant frequency of coil 30 b are lower than the sample losses at theresonant frequency of coil 30 a. In a typical implementation in whichthe proton channel is used for detection, the sample vessel and thecoils are preferably aligned such that the long transverse axis of thesample vessel coincides with the direction of the RF magnetic field ofthe proton coil. Such an alignment is also useful because sample lossesat the proton frequency are typically higher than at the resonantfrequencies of other nuclei. If a channel other than the proton channel(e.g. a carbon channel) is used for detection, the long transverse axisof the sample vessel can be aligned to coincide with the direction ofthe RF magnetic field of that channel (e.g. the direction of the RFmagnetic field of the carbon coil), even though the sample losses arenot highest at the carbon frequency.

FIG. 7-A is a top schematic view of probe 20, illustrating the stackingof the various components of probe 20. Moving outward in sequence fromsample vessel 22, FIG. 7-A shows shield 34 a, coil support 60, theconductive part of coil 30 a, an additional dielectric support 90, anadditional shield ring 92, support rungs 88, and the conductive part ofcoil 30 b. Dielectric support 90 preferably presses down the conductivepart of coil 30 a so as to secure the conductive part in place.Preferably, additional shield ring 92 can be slid longitudinallyrelative to the conductive part of coil 30 a, in order to tune thecapacitance of the coil and thus control the resonance frequency of coil30 a.

FIG. 7-B is a top schematic view of a probe 620 according to anotherembodiment of the present invention. Probe 620 includes a dewar 33 forkeeping a radio-frequency coil 630 at low temperature. Sample vessel 22is maintained at room temperature within an inner bore of dewar 33. Asshown, coil 630 may be a conventional saddle-shaped RF coilcharacterized by cylindrical symmetry. Since coil 630 is kept at a lowtemperature, the RF losses within coil 630 are typically much smallerthan the losses within the sample held in sample vessel 22. While anassembly including cylindrical RF coil(s) and a cylindrical dewar asshown in FIG. 7-B is suitable for use in a low-temperature probe, a lowtemperature probe can also benefit from the use oftransversely-elongated RF coils as described above, and atransversely-elongated Dewar having a matching shape.

FIG. 8 illustrates in a transverse view computed magnetic field linesfor a conventional coil and sample vessel characterized by cylindricalsymmetry. FIGS. 9-A and 9-B show magnetic field lines computed for asquashed rectangular vessel/coil arrangement as described above. FIG.9-A shows the magnetic field lines corresponding to the inner coil,while FIG. 9-B illustrates the magnetic field generated by the outercoil. The aspect ratio of the illustrated sample vessel is about 3.5:1.The homogeneity of the magnetic fields shown in FIGS. 9-A–B iscomparable to that achieved using the conventional geometry of FIG. 8.At the same time, the power dissipation in the arrangement of FIGS.9-A–B can be improved by a factor of 3 or more, as described below. FIG.9-C shows similar data for a squashed ellipsoidal vessel/coilarrangement as described above.

FIG. 10 shows an NMR nutation plot generated using a rectangular samplevessel having inner dimensions of 2 mm×7 mm, and matching rectangular RFcoil of the design shown in FIG. 5. The sample was 1% H₂O in 99% D₂O.The NMR probe and single coil were double tuned to proton and deuteriumlock. The deuterium signal was used for lock. The data shown is protondata at 800 MHz. The slow decay of the NMR signal amplitude withincreasing pulse duration indicates the RF magnetic field of the coil isrelatively uniform spatially.

The squashed vessel/coil arrangements described above allow reducingsample and/or coil losses, and thus allows higher quality (Q) factorsfor NMR measurement circuits. Improved NMR measurement performance isachievable when the physical cross-section of the sample is elongate inone transverse dimension relative to the other. A suitably-designedsaddle-shaped RF coil of substantially conformal transversecross-section, in combination with a squashed sample, can yield high Qvalues and consequent increases in signal-to-noise ratios. Theperformance improvement is particularly significant for samples havinghigh electrical conductivity, such as aqueous samples with significantsalt concentrations, and in other NMR systems in which sample losses areof particular concern, such as low-temperature NMR systems.

It will be clear to one skilled in the art that the above embodimentsmay be altered in many ways without departing from the scope of theinvention. A coil of the present invention need not have exactly fourlongitudinal conductors. Higher or lower numbers of longitudinalconductors may be used, depending on the particular desired coil shapeand number of coil turns. Accordingly, the scope of the invention shouldbe determined by the following claims and their legal equivalents.

1. An NMR probe for coupling an RF circuit to nuclei of a sample, saidprobe comprising, a sample containing vessel, said vessel having anextension along a symmetry axis, z, and a cross section σ(x,y)transverse to said axis z, wherein said cross section σ(x,y) has anaspect ratio x/y greater than 1.2, and an RF resonator comprising asaddle coil surrounding said sample containing vessel for coupling RFenergy between said sample in said sample vessel and said RF circuit. 2.The NMR probe of claim 1 wherein said saddle coil comprises a symmetryaxis z′ aligned coincident with said axis z and said saddle coilsupports an RF current distribution wherein said RF current isdistributed in the plane transverse to z′ in accord with a functionalshape S(x,y).
 3. The NMR probe of claim 2 wherein said shape S(x,y) hasan aspect ratio of substantially unity.
 4. The NMR probe of claim 2wherein said functional shape S(x,y) has an aspect ratio x/y in therange 1.2≦x/y≦10.
 5. The NMR probe of claim 4 wherein said functionalshape S(x,y) is geometrically similar to said cross section σ(x,y) andoriented with corresponding axes in alignment.
 6. The NMR probe of claim5 wherein said functional shape S(x,y) is generally ellipsoidal.
 7. TheNMR probe of claim 5 wherein said functional shape S(x,y) is generallyrectangular.
 8. The NMR probe of claim 5 wherein said functional shapeS(x,y) is generally rounded.
 9. The NMR probe of claim 2, wherein saidcross section σ(x, y) has a major axis, x and said RF saddle coilproduces an RF magnetic field substantially along said major axis x. 10.The method of coupling an RF circuit to a sample for NMR analysiscomprising the steps of a) imposing a unidirectional polarizing field onsaid sample, b) confining said sample to occupy a volume having a longaxis z, parallel with said polarizing field and said volume furthercharacterized by a cross section σ(x,y) transverse to z wherein x/y≧1.2,c) orienting a RF magnetic field coupled to said sample along said xaxis whereby the greater thickness of said sample is interposed alongthe direction of said RF magnetic field.